1. Field of the Invention
The present invention relates to a crystal oscillator having a base ground type oscillation circuit for use at a high frequency of a VHF band or the like. More particularly, it relates to a crystal oscillator in which a temperature characteristic of an oscillation frequency is improved to facilitate manufacturing.
2. Description of the Related Art
Heretofore, Colpitts crystal oscillation circuit is used in a temperature control type crystal oscillator.
This crystal oscillator has an operating temperature in a range of −20 to 70° C., and a temperature of an oven is set around 85° C. in order to assure 70° C.
Moreover, as an inflection point temperature of an SC cut crystal unit, a range of 80 to 90° C. is used.
Furthermore, when the operating temperature is in a range of −20 to 85° C., the temperature of the oven is set around 100° C. in order to assure 85° C.
In addition, as an inflection point temperature of an IT cut crystal unit, a range of 95 to 105° C. is used.
The crystal oscillation circuit mounted on a substrate generally has a negative temperature coefficient, and a temperature characteristic of the crystal unit is synthesized to obtain the temperature characteristic of the crystal oscillator.
The temperature of the oven is set to the inflection point temperature of the temperature characteristic of the crystal oscillator, whereby a frequency change due to a temperature fluctuation can be minimized. The set temperature at this time is slightly lower than the inflection point temperature of a single crystal unit, because the crystal oscillation circuit has the negative temperature coefficient.
Moreover, the temperature characteristic of the crystal oscillator including the crystal oscillation circuit needs to be measured before setting the temperature of the oven, and much time has been required in manufacturing the crystal oscillator.
It is to be noted that the concerned prior art is disclosed in Japanese Patent Application Laid-Open No. 2004-023568 (Patent Document 1).
In Patent Document 1, it is disclosed that in a base ground type oscillation circuit, a stationary capacitor and a thermistor having the negative temperature coefficient are connected in parallel with each other and inserted in a feedback loop of oscillation.
Moreover, a constitution and an operation of the base ground type oscillation circuit are described in A Wiley-Interscience Publication RF/Microwave Circuit Design for Wireless Applications authored by Ulrich L. Rohde, David P. Newkirk, p 756 to p 762.
However, the base ground type crystal oscillation circuit has a temperature coefficient larger than that of Colpitts crystal oscillation circuit, and the inflection point temperature comes close to the operating temperature range in the method of measuring the temperature characteristic of the oscillator to set the temperature of the oven as in the crystal oscillator using the Colpitts crystal oscillation circuit. There has been a problem that the method cannot cope with the circuit.